Technical Field
The present disclosure relates to a sensor device of a MEMS (Micro-Electro-Mechanical System) type, having a reduced sensitivity to stress, in particular to stress induced in a corresponding sensing structure via interaction with a corresponding package.
Description of the Related Art
The following description will make explicit reference, without this implying any loss of generality, to a MEMS accelerometer of a capacitive type.
MEMS accelerometers are known, with sensing axis in the horizontal plane, i.e., including microelectromechanical structures that sense accelerations acting along at least one direction parallel to a main plane of extension of the same structures and to the top surface of a corresponding substrate of semiconductor material (possibly also being able to detect further accelerations acting along a direction orthogonal to the same plane).
FIGS. 1a and 1b show by way of example a sensing structure of a known type, designated as a whole by 1, which belongs to a MEMS accelerometer with sensing axis in the plane.
The sensing structure 1 comprises: a substrate 2 made of semiconductor material, for example silicon, having a top surface 2a; and an inertial mass 3, which is made of conductive material, for example appropriately doped epitaxial silicon, and is arranged above the substrate 2, suspended at a certain distance from its top surface 2a. 
The inertial mass 3 is also usually referred to as “a rotor mass” or simply “rotor”, in so far as it is mobile as a result of the inertial effect, without, however, implying that the inertial mass 3 has a rotary movement. As described hereinafter, in this embodiment, the inertial mass 3 has, instead, a linear movement following upon sensing of an acceleration along the sensing axis.
The inertial mass 3 has a main extension in a horizontal plane xy, defined by a first horizontal axis x and by a second horizontal axis y orthogonal to one another, and substantially parallel to the top surface 2a of the substrate 2, and a substantially negligible dimension along an orthogonal axis z that is perpendicular to the aforesaid horizontal plane xy (and to the aforesaid top surface 2a of the substrate 2) and forms with the first and second horizontal axes x, y a set of three Cartesian axes xyz.
In particular, the first horizontal axis x coincides, in this case, with the sensing axis of the sensing structure 1.
The inertial mass 3 has a frame-like conformation, for example square or rectangular (or, in other words, it is shaped like a square or rectangular ring), in the aforesaid horizontal plane xy and has at the center a through opening, defining a window 4, which traverses it for an entire thickness.
The inertial mass 3 moreover has a plurality of holes 5, of very small size as compared to the aforesaid window 4, which traverse the mass 3 for its entire thickness, enabling, during the manufacturing process, its release (and its consequent suspended arrangement over the substrate 2) by chemical etching of an underlying sacrificial material, for example dielectric material (in a way not illustrated herein).
The sensing structure 1 further comprises a rotor anchorage structure 6 (so defined, since it is coupled, as will be described hereinafter, to the inertial mass, or rotor). The rotor anchorage structure 6 is set centrally within the window 4, parallel to the top surface 2a of the substrate 2, having a rectangular conformation in the horizontal plane xy and, in the example illustrated, a longitudinal extension along the first horizontal axis x.
The rotor anchorage structure 6 is fixedly coupled (anchored) to the substrate 2 by means of a rotor anchorage element 7, which extends like a pillar between the top surface 2a of the substrate 2 and the same anchorage structure 6. The rotor anchorage element 7 also has a rectangular conformation in the horizontal plane xy, with an extension smaller than that of the rotor anchorage structure 6, being located at a central portion of said rotor anchorage structure 6.
The inertial mass 3 is elastically coupled to the rotor anchorage structure 6 by means of elastic coupling elements 8, arranged within the window 4, between the inertial mass 3 and the rotor anchorage structure 6, on opposite sides of the rotor anchorage structure 6, along the direction of the first horizontal axis x.
In particular, the elastic coupling elements 8 are configured so as to enable movement of the inertial mass 3 along the first horizontal axis x with respect to the substrate 2, being compliant to deformations along the first horizontal axis x (and being substantially rigid with respect to deformations in different directions of the horizontal plane xy, or directions transverse to the same horizontal plane xy).
The inertial mass 3 moreover carries a number of mobile electrodes 9 (also referred to as “rotor electrodes”), fixedly coupled thereto (being integral, or made of a single piece, with the same inertial mass 3). The mobile electrodes 9 extend within the window 4 starting from the inertial mass 3. For example, the mobile electrodes 9 have a rectangular conformation in the horizontal plane xy and extend along the second horizontal axis y (in the example, two mobile electrodes 9 are present, extending on opposite sides with respect to the rotor anchorage structure 6, aligned along the second horizontal axis y). Like the inertial mass 3, the mobile electrodes 9 are suspended above the substrate 2, parallel to the top surface 2a of the same substrate 2.
The sensing structure 1 further comprises a number of first fixed electrodes 10a and second fixed electrodes 10b (also referred to as “stator electrodes”), which are set within the window 4, on opposite sides of a respective mobile electrode 9 in the direction of the first horizontal axis x, and face the respective mobile electrode 9 in the horizontal plane xy.
The first and second fixed electrodes 10a, 10b have in the example a rectangular conformation in the horizontal plane xy, with extension along the second horizontal axis y, in a direction parallel to the mobile electrodes 9.
The first and second fixed electrodes 10a, 10b are moreover rigidly coupled to the substrate 2 by means of respective stator anchorage elements 12a, 12b, which extend in the form of pillars between the top surface 2a of the substrate 2 and the same first and second fixed electrodes 10a, 10b. The stator anchorage elements 12a, 12b also have a rectangular conformation in the horizontal plane xy, with an extension smaller than that of the corresponding fixed electrodes 10a, 10b, being located at a central portion of the same fixed electrodes 10a, 10b. 
The first fixed electrodes 10a and the second fixed electrodes 10b are biased (for example, through the corresponding stator anchorage element 12a, 12b) at a respective biasing voltage, different from the biasing voltage at which the mobile electrodes 9 are biased (for example, via a conductive path that includes the rotor anchorage element 7, the rotor anchorage structure 6, the elastic coupling elements 8, and the inertial mass 3, in this case all including electrically conductive material).
During operation, an acceleration component acting in the horizontal plane, in the example along the first horizontal axis x, determines displacement, by the inertial effect, of the inertial mass 3 from a resting position, along the same first horizontal axis x, and moving of the mobile electrodes 9 towards the first fixed electrodes 10a (or the second fixed electrodes 10b) and away from the second fixed electrodes 10b (or the first fixed electrodes 10a).
Consequently, a differential capacitive variation occurs between a first sensing capacitor C1, with plane and parallel faces, formed between the mobile electrodes 9 and the first fixed electrodes 10a, and a second sensing capacitor C2, also with plane and parallel faces, formed between the mobile electrodes 9 and the second fixed electrodes 10b. This capacitive variation is proportional to the value of the acceleration acting on the sensing structure 1.
The MEMS accelerometer further comprises an appropriate electronic reading circuit (so-called ASIC—Application-Specific Integrated Circuit), electrically coupled to the sensing structure 1, which receives at its input the aforesaid differential capacitive variation of the sensing capacitors C1, C2, and processes it so as to determine the value of acceleration, for generation of an electrical output signal (which is supplied to the outside of the MEMS accelerometer, for subsequent processing operations).
The aforesaid electronic reading circuit and the sensing structure 1 are typically provided in respective dies of semiconductor material, which are enclosed within a package, which protects the same dies, moreover providing an interface for electrical connection towards the outside. In so-called “substrate-level package” solutions, the package is obtained by means of one or more base and cap layers, which are directly coupled to the dies of the MEMS device, constituting the mechanical and electrical interface thereof towards the outside.
FIG. 2 is a schematic representation of a MEMS accelerometer 13, which includes a package 14 housing: a first die 15a, in which the ASIC electronic circuit is provided; a second die 15b, in which the sensing structure 1 is provided (including the substrate 2, above which the inertial mass 3, the mobile electrodes 9, and the first and second fixed electrodes 10a, 10b are provided), set stacked on the first die 15a; and a cap 15c, set on the stacked arrangement of the first and second dies 15a, 15b. 
The package 14 further comprises: a base layer 16, which supports the aforesaid stacked arrangement of the first and second dies 15a, 15b and a cap 15c, and having an external surface 16a, which constitutes an external surface of the package 14 and carries electrical connection elements for connection to the outside, for example in the form of electrical connection pads 18; and a coating region 19, made for example of epoxy resin, which surrounds the aforesaid stacked arrangement of the first and second dies 15a, 15b and cap 15c, and defines lateral and top external surfaces of the package 14, designed to be in contact with the external environment.
The present applicant has realized that the sensing structure 1 previously described may be subject to significant measurement errors in the case where stress and deformations occur, for example as the temperature or environmental conditions vary, or on account of mechanical stresses.
In particular, the package of a microelectromechanical sensor undergoes deformations as the temperature varies, due to different coefficients of thermal expansion and to different values of Young's modulus of the different and variegated materials of which it is made, possibly causing corresponding deformations of the substrate 2 of the sensing structure 1 housed therein. Similar deformations may arise due to ageing of the materials, or to specific stress induced from outside, for example during soldering of the package on a printed-circuit board, or due to absorption of humidity by the materials of the same package.
As shown schematically in FIG. 3a, in the presence of deformations of the substrate 2, for example as a result of a thermal stress due to a positive temperature gradient (ΔT>0), a swelling of the top surface 2a of the substrate 2 may for example occur (FIG. 3a markedly shows this deformation), which can entail moving of the fixed electrodes 10a, 10b away from the corresponding mobile electrode 9, with respect to an initial condition at rest, in the absence of external accelerations.
In particular, in FIG. 3b the distance at rest and in the absence of deformations is designated by g0, and the displacements with respect to the condition at rest due to the deformation of the substrate 2 (which are variable as a function of the temperature, or in general of all those effects that are able to induce deformations in the substrate 2) are designated by x1,ΔT and x2,ΔT.
These displacements entail the following variations of the values of the sensing capacitances C1, C2, which are undesired, in so far as they are not associated to the acceleration to be detected:
                    C                  1          ,                      Δ            ⁢                                                  ⁢            T                              =                                    ɛ            0                    ·          A                                      g            0                    +                      x                          1              ,                              Δ                ⁢                                                                  ⁢                T                                                          ;    and            C              2        ,                  Δ          ⁢                                          ⁢          T                      =                            ɛ          0                ·        A                              g          0                +                  x                      2            ,                          Δ              ⁢                                                          ⁢              T                                          
Likewise, with reference to FIGS. 4a and 4b, a negative temperature gradient (ΔT<0) entails, on account of a depression of the top surface 2a of the substrate 2, the following undesired variations of the values of the sensing capacitances C1, C2, on account, this time, of an approach of the fixed electrodes 10a, 10b to the corresponding mobile electrode 9, with respect to the initial condition at rest:
                    C                  1          ,                      Δ            ⁢                                                  ⁢            T                              =                                    ɛ            0                    ·          A                                      g            0                    -                      x                          1              ,                              Δ                ⁢                                                                  ⁢                T                                                          ;    and            C              2        ,                  Δ          ⁢                                          ⁢          T                      =                            ɛ          0                ·        A                              g          0                -                  x                      2            ,                          Δ              ⁢                                                          ⁢              T                                          
These capacitive variations hence cause an undesired modification (the so-called “drift” or “offset”) of the output signal at rest supplied by the MEMS accelerometer, referred to as “zero-g level”, and a consequent error in acceleration detection.
In order to overcome the above drawback, a wide range of solutions have been proposed, which are not, however, altogether satisfactory.
Some solutions envisage optimization of the acceleration-sensing structure.
For example, US 2011/0023604 discloses a sensing structure, wherein positioning of the rotor and stator anchorages is optimized in order to reduce the drifts of the electrical parameters due to deformations of the substrate.
However, this solution, although advantageous, relates to a z-axis accelerometer, of a vertical type, i.e., designed for sensing accelerations orthogonal to the horizontal plane of main extension of the corresponding rotor mass.
Other solutions envisage optimization of the corresponding package. For example, use of a ceramic substrate, having a reduced sensitivity to deformations, has been proposed for this purpose.
However, this solution entails greater issues in the manufacturing process and in general higher costs. Moreover, the size of ceramic packages is generally greater than that of traditional packages made of plastic material.